1. Field of the Invention
The present invention relates to an exhaust gas purification system used in an internal combustion engine for purifying exhaust gas.
2. Description of the Related Art
As a technique for purifying exhaust gas discharged from an internal combustion engine, an exhaust gas purification system (a plasma reactor) has been conventionally proposed, using plasma generated by applying a high voltage to a pair of opposed electrodes. For example, Japanese Patent Application Laid-open No. 2001-295629 discloses a filtration type apparatus disposed in a flow path of the exhaust gas, which is formed by sandwiching a fibrous filter between a pair of metallic nets constituting an electrode. According to this apparatus, particulate matters (PM) in the exhaust gas caught by the fibrous filter is made to be radicals by energy of plasma generated by the application of high voltage between both the electrodes, wherein HC changes to H2O and CO2 and a portion of PM burns to be incinerated. Such a filtration type apparatus has a drawback in that a pressure loss increases due to the exhaust resistance when the exhaust gas passes the filter.
On the other hand, as disclosed in Japanese Patent Application Laid-open No. 2002-21541, a straight-flow type or an electrostatic attraction type exhaust gas purification system is proposed, including a tubular outer circumferential electrode for catching particulate matters and a central electrode extending along an axis thereof, wherein plasma for cleaning the exhaust gas passing therethrough is generated by applying high voltage between both the electrodes. In this apparatus, PM in the supplied exhaust gas are charged due to the electric discharge from the central electrode, attracted to the outer circumferential electrode charged to have a polarity opposite thereto, and deposited thereon. The deposited PM are incinerated by heat and chemical reaction caused as high voltage is applied between both the electrodes. HC, CO and NOx in the exhaust gas react with oxygen in the exhaust gas to be harmless N2, CO2, H2O or others by energy of plasma formed between the central electrode and the outer circumferential electrode. In comparison with the above-mentioned filtration type apparatus, the electrostatic attraction type apparatus has an advantage in that a pressure loss due to the exhaust pressure decreases.
According to the electrostatic attraction type apparatus, however, there is a problem in that incompletely burnt PM are liable to generate as the deposit thereof increases and particularly, when an attracting honeycomb structure body is provided in the outer circumferential electrode, ash remaining after the combustion treatment causes the increase in pressure loss due to the reduction of a cell cross-section (opening area), resulting in the lowering of processing performance. A particle diameter of the attracted PM increases as the deposition progresses by the mutual bonding of the respective particulates via HC component, H2O component or others, whereby there is a risk in that PM may be peeled off from the attracting surface before it is processed by a high voltage and released downstream of the apparatus.
An object of the present invention is to provide means capable of restricting the discharge of non-treated PM so that the drawbacks in the conventional electrostatic type apparatus are minimized.